1. Field of the Invention
The present invention relates to a high voltage capacitor used for a low-pass filter circuit or the like in a magnetron oscillator circuit of an electronic cooking range.
2. Description of the Related Art
FIG. 22 shows an example of the magnetron oscillator circuit of an electronic cooking range. This kind of circuit is provided with a constant (K) filter circuit comprising capacitors 6 and 8 and coils 10 and 12 to prevent the external conduction and radiation of high frequency noise generated from a magnetron 14. In this case, voltage applied to the capacitors 6 and 8 ordinarily includes heater voltage (e.g., 6,3 V rms) and anode voltage (e.g., Dc 3 to 6 KV) of the magnetron 14. From the property of the magnetron 14 and a step-up transformer (magnetic leakage transformer) 2, since an inrush pulse and a rising pulse of 10 to 20 KV.sub.o -p is generated during the oscillation of magnetron 14, the capacitors 6 and 8 require a high breakdown voltage durability of 20 KV and the electronic cooking range requires an accurate cooking-heating cycle when used. Hence, a relatively complicated capacitor as shown in FIG. 23 has hitherto been used as the capacitors 6 and 8 in the circuit shown in FIG. 22.
Specifically, the high voltage capacitor includes perforating conductors 24a and 24b extending through through-bores 29a and 29b defined in a substantially elliptical capacitor unit 28 (see FIG. 24). The outside of the unit 28 is covered by insulating cases 18 and 28 to be molded with insulating resin 38. Split electrodes 32a and 32b at one end face of the capacitor unit 28 are soldered to the perforating conductors 24a and 24b through cap terminals 26a and 26b, an all-out electrode 34 at the other end face being soldered to ground terminal 22. Hence, the capacitors are disposed between the perforating conductors 24a and 24b and the ground terminal 22.
However, the high voltage capacitor shown in FIG. 23 is subject to the following problems:
(I) In order to suppress stress in the insulating resin 38 at the through-bores 29a and 29b of the capacitor unit 28, the perforating conductors 24a and 24b in the insulating resin 38 are required to be covered with tubes 36a and 36b comprising flexible material (e.g., silicone rubber) attributing to a high manufacturing cost. The reason for this is that since the perforating conductors 24a and 24b are superior to a porcelain dielectric 30 with respect to adhering to insulating resin 38, if the tubes 36a and 36b are omitted, the residual stress or stress due to a temperature change in insulating resin 38 results in the insulating resin 38 peeling from the porcelain dielectric 30, thereby lowering the breakdown voltage durability.
(II) Since the coefficient of linear expansion of the porcelain dielectric 30 at the capacitor unit 28 is smaller by about one digit than that of the insulating casing 18 or the insulating resin 38, the porcelain dielectric 30 and insulating resin 38 are liable to cause a peel-off under the changing thermal conditions. As a result, the breakdown voltage durability lowers.
(III) The number of parts is larger and construction is complicated, whereby the capacitor is hard to manufacture and expensive to produce.
In order to avoid the above-mentioned problems, it is required to accurately set the dimension of a gap between the perforating conductors 24a and 24b and the inner electrodes 31a and 31b and to accurately maintain a soldering condition such as the heating temperature of solder, resulting in difficulties pertaining to dimension management or condition management.